It is well known to prepare caps, crowns, bridges, inlays and other dental restorations from alloy castings, to which a porcelain or like ceramic surface is applied and then bonded, as by firing, in order to reproduce the function, colour and shape of natural teeth.
The alloys used in such work are expected to satisfy a number of requirements. Thus, in addition to being physiologically acceptable for long-term use in the mouth, the alloys should exhibit sufficient compressive strength in order to withstand the force exerted thereon during mastication; a Vickers Hardness of at least 150 is generally required. The alloy should also have a liquidus temperature which is low enough for it to be cast with the equipment normally found in dental laboratories, whilst exhibiting a solidus which is sufficiently high to withstand the ceramic-firing temperature and to permit the alloy to be soldered, if necessary. The alloy should generally, therefore, exhibit a liquidus of not more than 1400.degree. C. and a solidus of at least 1100.degree. C.
The alloys ideally should also have sufficient tensile elongation (in general, at least 2% and preferably at least 4%) to permit marginal adjustment after placement in the mouth and should also have a coefficient of thermal expansion such that the ceramic coating is subjected to moderate compression as it cools from the elevated temperature at which it was fired, since this improves the bond strength of the resultant restoration or prosthesis. In general, a coefficient of thermal expansion of from 0.66 to 0.72 percent at 500.degree. C. is desirable for compatibility with commercially available dental porcelains.
Needless to say, the alloy should also provide a firm bond to the ceramic applied thereto and should not give rise to discoloration of the ceramic surface.
Until recently, alloys of gold and platinum had been used as dental alloys. However, as the prices of these metals have risen, alternatives have been sought.
Palladium/silver alloys have been developed which have exhibited the required workability and porcelain-bonding properties but these have proved unsatisfactory, owing to their tendency to discolour the dental porcelain applied thereto.
GB-A-2,118,971 discloses a dental alloy consisting substantially of 50 to 85% palladium; 5 to 40% copper, cobalt or a mixture thereof; 1 to 15% gallium; up to 5% of a modifier selected from nickel, gold, indium, ruthenium, tin and mixtures thereof; up to 1% boron; and up to 0.5% of a grain refiner selected from rhenium, iridium and mixtures thereof (the percentages being by weight). This British patent specification teaches (page 2, lines 31-33) that an amount of modifier metal in excess of 5% would adversely affect the balance of properties of the alloy and should not be employed.
US-A-4,608,229, however, teaches that it is possible to incorporate the modifier metal into certain palladium/copper/gallium alloys in an amount in excess of 5%, whilst still obtaining a low-cost dental alloy casting having a fracture-resistant, non-dendritic structure with acceptable Vickers Hardness, good ductility and excellent porcelain bond strength. Specifically, this US patent discloses an alloy that consists essentially of 60 to 85% palladium, 5 to 20% copper, 3 to 15% gallium and, as modifiers, 0.5 to 2% gold, 3.2 to 6% indium, 0.005 to 0.02% ruthenium, 0 to 2% tin and 0 to 2% nickel, the sum of the concentrations of the modifiers being greater than 5.5% and up to 8.5% (the percentages being by weight).
It has been found, however, that alloys within the composition defined in US-A-4,608,229 have a tendency to cause the formation of bubbles in the dental ceramic during the firing stage. Although this tendency can be lessened by the application of a high level of expertise by the dental technician, it would clearly be desirable to develop an alloy which does not show this tendency and which, therefore, places less of a demand upon the skill of the dental technician.
The alloys disclosed in US-A-4,608,229 have also been found to exhibit variable workability.